This article challenges exclusively rationalist accounts of and offers a complementary explanation for the emergence of liberal trade policy in the Kennedy administration. I draw on recent insights in constructivist institutionalism to emphasize the need to take agency seriously in institutionalist research. Using archival records, I analyze the decisive role Kennedy's advisers played as carriers of ideas in advocating for liberal trade policy by ‘constructing the national interest’, thus convincing a reticent president to support attempts aimed at achieving closer economic integration, culminating in the Trade Expansion Act of 1962. Insights from their role as advisers can help in specifying the role of agency in the ideas and institutional change literature, through strategic action which shaped a political leader's belief and put political issues on the agenda. By grasping agency in terms of making ideas actionable, an important step is taken in advancing endogenous approaches of institutional change.